Advanced magnetic-tape cartridges hold multiple terabytes of data. The recording densities necessary to achieve this capacity require that the read elements have nearly or identically the width of the read/write head elements. This presents an environmentally-induced density-limitation as magnetic tape is exposed to changes in temperature and humidity. If a tape is written in a climate having a high temperature and a low humidity, the tape will be wider (expanded along the Y-axis) than if it is later read at a low temperature and low humidity. Thus, when the tape is read, the read elements may read data from adjacent tracks resulting in a poor signal-to-noise ratio.
Servo patterns may be used to determine the extent of the environmental effects, i.e., the extent to which a read/write head is reading off the track. In timing-based servo (TBS) systems, recorded servo patterns consist of magnetic transitions with two different azimuthal slopes. The read/write head position is derived from the relative timing of pulses, or dibits, generated by a narrow servo element reading the servo pattern. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position using pulse-position modulation (PPM). A specification for the servo format in current tape drives is provided by the linear tape-open (LTO) format. The completed format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified, can be found on the Internet at ultrium.com. Traditionally, the detection of LPOS information bits is based on the observation of the arrival times of the shifted dibit peaks within the servo bursts at the servo reader output.
TBS technology, which was developed specifically for linear tape drives and is also used in all LTO tape drive products, provides the basic structure of a servo frame, consisting of four servo bursts. The signal obtained by reading the servo pattern is used to extract essential servo-channel parameters such as tape velocity, read/write head Y-position, and LPOS information, which is encoded by using PPM.